The present invention is directed to a titration apparatus and method therefor and more specifically to the on-line measurement of the available alkali in a Kamyr or batch digester being used for processing paper pulp.
As stated in a paper entitled "Computer Control in Pulp and Paper 1961-1969" by Donald B. Brewster and Andrew K. Bjerring in the Proceedings of the IEEE, Volume 58, No. 1, Jan. 1970, page 51, "at the present time there is no effective commercial on-line instrument available for measuring K number" (in a Kamyr digester). "Manual tests on samples taken from the blow line or washer (of the digester) are performed, usually once per hour." The purpose of the pulp digester is, of course, to convert the wood into fiber. This is achieved in the digester by treating the wood chips with a white liquor which consists of a solution of sodium sulfide and sodium hydroxide. Both high heat and pressure are present in the digester to provide delignification. Lignin accounts for about 30% of the wood and it is removed to allow easy separation and bleaching of the cellulose fibers. The usual measure of the extent of delignification is the "K number"; the lower the K number the lower the residual lignin. An alternate to the measure of K number is the measure of the alkalinity of the liquor in the digester indicating the extent of the reaction between the alkali and the lignin.
With an on-line measurement of K number or alkalinity much better control of the digester is possible. At the present time as discussed on page 55 of the Brewster article the most feasible method of K number regulation is to use feedforward control and keep the following five factors constant: alkali to wood ratio, liquor to wood ratio, temperature, residence time and chip level control. In order to stabilize this feedforward control the one hour sample of the blow line is utilized as a feedback control. It is apparent that because of the long time delay involved to obtain a K number sample the entire control scheme is unsatisfactory. To remedy the foregoing a Swedish company under the name ASEA utilizes an on-site titrator to determine alkalinity with a "mechanical man" to obtain the sample. This is not an effective on-line measurement since the time required is over ten minutes. Also the moving parts of the mechanical man, especially under the high pressure and temperature conditions in a typical digester, make the design, operation, and maintenance of such a system impractical.